. Outlines of zoology. platesof tortoise shell, which, though very hard, are not withoutsensitiveness, numerous nerves ending upon them. The quadrate is immovably unitedwith the skull. There is only alower. temporal arcade. The jawsare covered by a horny sheath, andare without teeth, though hints ofthese have been seen in some em-bryos. There is a single anteriornasal opening. The scapular archis internal to the ribs. The limbsare pentadactyl, but often in theform of paddles. The average life of Chelonians issluggish. Perhaps this is in partdue to the way in which the ribsare lost in the carap
. Outlines of zoology. platesof tortoise shell, which, though very hard, are not withoutsensitiveness, numerous nerves ending upon them. The quadrate is immovably unitedwith the skull. There is only alower. temporal arcade. The jawsare covered by a horny sheath, andare without teeth, though hints ofthese have been seen in some em-bryos. There is a single anteriornasal opening. The scapular archis internal to the ribs. The limbsare pentadactyl, but often in theform of paddles. The average life of Chelonians issluggish. Perhaps this is in partdue to the way in which the ribsare lost in the carapace, for thisCarapace of must tend to make respiration lessactive. The lungs are divided intoThe dark contours are those of a number of compartments. the bony pieces; the lighter rri-i 7 7 ^ j • 77 contours are those of the scales J-tie cloacal aperture ts usuallywhich have been removed. longitudinal, never transverse ; the copulatory organ is are oviparous. The eggs have firm, usually calcareous, Fig. 331 tortoise. Some Peculiarities in the Skeleton of Chelonia The (10) dorsal vertebrae are without transverse or articular processes,and along with the ribs are for the most part immovably fused in thecarapace. The tail and neck are the only flexible regions. There aretwo sacral vertebrae. The greater part of the dorsal shield is due to a coalescence of eightribs with eight costal plates derived from the dermis. SKELETON OF CHELONtA. 615 Similarly, the median pieces are the result of fusion between mediandermal bones and the neural spines of the vertebra. The plastronusually consists of nine dermal bones, and the three anterior piecesperhaps represent clavicles and interclavicle (or episternum). The eight cervical vertebrje have at most little rudiments of ribs, areremarkably varied as regards their articular faces, and give the neckmany possibiUties of motion. There are no lumbar vertebrae. The bones of the skull are immovably united; there is only a lower
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
